Activation of transducin gtpase by two proteins from the rgs family

Purpose. GTPase activity of transducin in rod outer segments (ROS) is know to be stimulated by a cooperative action of the y-subunit of cGMP phosphodiesterase (PDE.,) and another unidentified protein factor tightly associated with ROS membranes. We determined whether the proteins from the RGS family can stimulate GTPase activity of transducin in order to evaluate the hypothesis that the unidentified factor might be a member of the RGS family. Methods. Various kinetic approaches were used to study the effects of RGS4 and GAIP proteins on the rate of transducin GTPase. These measurements were performed in the presence or absence of PDEV. Results. Both RGS4 and GAIP are able to stimulate transducin GTPase. RGS4. being at least 5-fold more potent than GAIP, stimulates the rate of transducin GTPase by two orders of magnitude. However, the mechanism of their action is different from transducin GTPase stimulation in ROS: neither RGS4 nor GAIP require PDEV for activating transducin. Rather, PDEy acts as a partial competitive inhibitor of transducin GTPase activation by RGS proteins. Conclusion. Our observations indicate that GTPase activity of transducin can be activated by at least two distinct mechanism, one based on the action of RGS proteins alone and another involving the cooperative action of the effector enzvme and another orotein.